High-capacitance photodiode

ABSTRACT

A monolithic photodetector including a photodiode, a precharge MOS transistor, a control MOS transistor, a read MOS transistor, and a transfer MOS transistor, the photodiode and the transfer transistor being formed in a same substrate of a first conductivity type, the photodiode including a first region of the second conductivity type formed under a second region of the first conductivity type more heavily doped than the first region, and above a third region of the first conductivity type more heavily doped than the substrate, the first region being the source of the second conductivity type of the transfer transistor, the second and third regions being connected to the substrate and being at a fixed voltage.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to monolithic image sensorsintended for being used in image pick up devices such as, for example,cameras, camcorders, digital microscopes, or digital photographiccameras. More specifically, the present invention relates to imagessensors based on semiconductors including a single storage andphotodetection element.

[0003] 2. Discussion of the Related Art

[0004]FIG. 1 partially illustrates a portion of a line of an array of animage sensor. With each line in the array are associated a prechargedevice and a read device. The precharge device is formed of an N-channelMOS transistor M1, interposed between a supply rail Vdd and anend-of-line node I. The gate of precharge transistor M1 is adapted toreceive a precharge control signal Rs. The read device is formed of theseries connection of two N-channel MOS transistors. The drain of a firstone of these read transistors, hereafter, M2, is connected to supplyrail Vdd. The source of the second read transistor M3 is connected toinput terminal P of an electronic processing circuit. The gate of firstread transistor M2 is connected to end-of-line node I. The gate ofsecond read transistor M3 is adapted to receiving a read signal Rd. Theline includes a plurality of photodiodes. In FIG. 1, a single photodiodeD2, the closest to node I, is shown. Node I is associated with a chargestorage diode D1. The anode of each diode D1, D2 . . . is connected to areference supply rail or circuit ground GND. The cathode of diode D1 isdirectly connected to node I. Then, the cathodes of two consecutivediodes D1 and D2 are separated by a charge transfer N-channel MOStransistor, such as transistor M4 between diodes D1 and D2. The gate oftransfer transistor M4 is adapted to receive a charge transfer signal T.The operation of this circuit is the following.

[0005] A photodetection cycle starts with a precharge phase during whicha reference voltage level is imposed to diode D1. This precharge isperformed by maintaining second read transistor M3 off and by turning onprecharge transistor M1. Once the precharge has been performed,precharge transistor M1 is turned off. Then, the system is maintained assuch, all transistors being off.

[0006] A given time after the end of the precharge, the state at node I,that is, the real reference charge state of diode D1 is read. Toevaluate the charge state, second read transistor M3 is turned on for avery short time δt.

[0007] The cycle continues with a transfer to node I of thephotogenerated charges, that is, the charges created and stored in thepresence of a radiation, in upstream photodiode D2. This transfer isperformed by turning transfer transistor M4 on.

[0008] Once the transfer is over, transistor M4 is turned off andphotodiode D2 starts photogenerating and storing charges which will besubsequently transferred back to node I.

[0009] Simultaneously, at the end of the transfer, the new charge stateof diode D1 is read. The output signal transmitted to terminal P thendepends on the channel pinch of first read transistor M2, which directlydepends on the charge stored in the photodiode.

[0010] Once the reading is over, transistor M3 is turned off and thecycle starts again with a precharge of diode D1.

[0011]FIG. 2 illustrates, in a partial simplified cross-section view, amonolithic forming of the assembly of a photodiode D2 and of transfertransistor M4 of FIG. 1. These elements are formed in a same active areaof a semiconductor substrate 1 of a first conductivity type, forexample, of type P, which is lightly doped (P−). This substrate forexample corresponds to an epitaxial layer on a silicon wafer. The activearea is delimited by field insulating layers 2, for example made ofsilicon oxide (SiO2) and corresponds to a well 3 of the sameconductivity type as underlying substrate 1, but more heavily doped.Above the surface of well 3 is formed an insulated gate structure 4possibly provided with lateral spacers. On either side of gate 4, at thesurface of well 3, are located source and drain regions 5 and 6 of theopposite conductivity type, for example, N. Drain region 6, to the rightof gate 4, is heavily doped (N+). Source region 5 is formed on a muchlarger surface area than drain region 6 and forms with underlyingsubstrate 3 the junction of photodiode D2. Gate 4 and drain 6 are solidwith metallizations (not shown) which enable putting in contact theseregions with transfer control signal T and the gate of transistor M2(node I), respectively. The structure is completed by heavily-dopedP-type regions 8 and 9 (P+). Regions 8 and 9, underlying areas 2, areconnected to the reference or ground voltage via well 3 and substrate 1.Photodiode D2 is of the so-called completely depleted type and includes,at the surface of its source 5, a shallow very heavily-doped P-typeregion 7 (P+). Region 7 is in lateral (vertical) contact with region 8.It is thus permanently maintained at the reference voltage level.

[0012]FIG. 3 illustrates the voltage levels of the various regions ofFIG. 2. The curve in full line illustrates the system state after atransfer, after transistors M4 and M1 have been turned on. Photodiode D2reaches a so-called depletion quiescent level VD determined by the solecharacteristics of the diode, as will be explained in further detailhereafter. Heavily-doped P-type regions 7, 8, and 9 are continuouslymaintained at the reference or ground voltage, for example, 0 V. Channelregion 3 of transistor M4 is at a voltage Vdd-VT. Region 6 (node I) isat Vdd. When transistor M4 is off, its channel region switches to 0 V(dotted line). Region 5 of photodiode D2 then forms a voltage well,which fills up (dotted line) according to the lighting of thisphotodiode. Then, when transistor M4 turns back on (transistor M1 beingmaintained off), the charges accumulated in region 5 are transferred toregion 6, the voltage of which varies (dotted line).

[0013] The use of a photodiode D2 (FIGS. 1, 2) of completely depletedtype enables suppressing or eliminating any noise at the level ofphotodiode D2. For this purpose, the doping profiles are chosen so thatregion 5, pinched between surface region 7 and underlying substrate 3,is depleted. Voltage VD in depletion state, that is, in the absence ofany radiation, is adjusted only by the doping of regions 7, 5, and 3.This voltage is chosen, as illustrated in FIG. 3, at a value smallerthan the channel voltage of transfer transistor M4 during the transferof charges from the photodiode to node I.

[0014] A disadvantage of such photodiodes is the fact that theircapacitance is relatively low. This is particularly disadvantageous withthe decrease in supply voltages Vdd in CMOS technologies. Indeed, asupply reduction from 5 to 3.3 V imposes setting the depletion voltageVD of photodiode D2 to approximately 1 V. The capacitance associatedwith this diode then is too low to obtain sufficient dynamics.

SUMMARY OF THE INVENTION

[0015] The present invention thus aims at providing a photodiode havingan increased capacitance.

[0016] To achieve this and other objects, the present invention providesa photodetector made in monolithic form, of the type including aphotodiode, a precharge MOS transistor, a control MOS transistor, a readMOS transistor, and a transfer MOS transistor, the photodiode and thetransfer transistor being formed in a same substrate of a firstconductivity type, the photodiode including a first region of the secondconductivity type formed under a second region of the first conductivitytype more heavily doped than the first region, and above a third regionof the first conductivity type more heavily doped than the substrate,the first region being the source of the second conductivity type of thetransfer transistor, the second and third regions being connected to thesubstrate and being at a fixed voltage.

[0017] According to an embodiment of the present invention, thephotodetector further includes a well of the first conductivity type,more heavily doped than the substrate, in which the first region isformed.

[0018] According to an embodiment of the present invention, the firstconductivity type is type P and the second conductivity type is type N.

[0019] According to an embodiment of the present invention, thesubstrate, the well, and the second and third regions are maintained toa reference voltage of the circuit.

[0020] According to an embodiment of the present invention, thephotodetector includes chains of photodiodes connected together bytransfer transistors.

[0021] According to an embodiment of the present invention, the thirdregion has a thickness such that it is an integral part of the spacecharge area between the first and third regions.

[0022] The foregoing objects, features and advantages of the presentinvention will be discussed in detail in the following non-limitingdescription of specific embodiments in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a schematic electric diagram of an image sensor;

[0024]FIG. 2 is a partial simplified cross-section view of a portion ofthe circuit of FIG. 1 made in monolithic form according to prior art;

[0025]FIG. 3 illustrates voltage levels in the structure of FIG. 2;

[0026]FIG. 4 illustrates, in a partial simplified cross-section view,the forming of a portion of the circuit of FIG. 1 according to anembodiment of the present invention;

[0027]FIGS. 5A and 5B illustrate doping levels in the structures ofFIGS. 2 and 4, respectively.

DETAILED DESCRIPTION

[0028] The same elements have been designated with the same referencesin the different drawings and, further, as usual in the representationof integrated circuits, FIGS. 2 and 4 are not drawn to scale.

[0029] A feature of the present invention is to modify the structure ofeach photodiode in the way described hereafter in relation with FIG. 4.

[0030] According to the present invention, a P-type buried region 30 isformed under the bottom of cathode region 5 of diode D2. Buried region30 thus is of the same conductivity type as peripheral well 3 and assubstrate 1 (anode of diode D2). However, buried region 30 is moreheavily doped than well 3. The surface of source 5 includes aheavily-doped P-type shallow region 7. Buried region 30 is more lightlydoped than region 7.

[0031] The operating cycle of the photodiode according to the presentinvention is similar to that described previously. The variation of thevoltage levels upon transfer from diode D2 to node I also remainsunchanged.

[0032] However, the photodiode according to the present inventionadvantageously has an increased capacitance. Indeed, the presence ofburied region 30 reduces the extent of the space charge area in P-typeregion. By providing a region 30 more heavily doped than peripheral well3, a non-negligible capacitance, greater than for a conventionalhomologous junction between a source region 5 and well 3 or substrate 1,will be obtained. The presence of region 30 adds a charge storagecapacitance in parallel with the capacitance associated with thepresence of surface region 7. Accordingly, the maximum charge that canbe stored in a photodiode D2, for a same supply voltage level, isincreased with respect to a conventional photodiode.

[0033] Further, the present of buried region 30 enables ensuringdepletion of source region 5 while the doping level thereof is increasedwith respect to a conventional structure. Such a doping increase ofregion 5 causes an increase in the capacitance associated with theregion 5/region 7 junction. FIGS. 5A and 5B illustrate the doping levelsaccording to thickness e in a conventional photodiode such asillustrated in FIG. 2 and in a photodiode according to the presentinvention such as illustrated in FIG. 4, respectively.

[0034] Preferably, the thickness of region 30 is chosen so that, in thequiescent state, region 30 is completely depleted. In other words, thethickness of region 30 is chosen so that region 30 is an integral partof the space charge area between region 5 and region 30.

[0035] The increase in the capacitance associated with photodiode D2,and thus in the maximum charge that can be stored, is particularlyadvantageous. Indeed, increasing the number of charges that can bestored amounts to increasing the useful signal proportion. The dynamics,that is, the signal-to-noise ratio, is thus considerably improved. Inpractical words, this means that the contrast is improved and that thephotodetectors can operate in a wider lighting range. Indeed, withconventional devices, the saturation, that is, the maximum amount ofcharges that can be reached, is reached very quickly.

[0036] Of course, the present invention is likely to have variousalterations, modifications, and improvements which will readily occur tothose skilled in the art. In particular, those skilled in the art willknow how to adjust the doping levels and types to the desiredperformances and to the materials used according to the constraints of aspecific manufacturing technology.

[0037] Such alterations, modifications, and improvements are intended tobe part of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

What is claimed is:
 1. A monolithic photodetector including aphotodiode, a precharge MOS transistor, a control MOS transistor, a readMOS transistor, and a transfer MOS transistor, the photodiode and thetransfer transistor being formed in a same substrate of a firstconductivity type, wherein the photodiode includes a first region of thesecond conductivity type formed under a second region of the firstconductivity type more heavily doped than the first region, and above athird region of the first conductivity type more heavily doped than thesubstrate, the first region being the source of the second conductivitytype of the transfer transistor, the second and third regions beingconnected to the substrate and being at a fixed voltage.
 2. Thephotodetector of claim 1, further including a well of the firstconductivity type, more heavily doped than the substrate, in which thefirst region is formed.
 3. The photodetector of claim 1, wherein thefirst conductivity type is type P and the second conductivity type istype N.
 4. The photodetector of claim 2, wherein the substrate, thewell, and the second and third regions are maintained to a referencevoltage of the circuit.
 5. The photodetector of claim 1, includingchains of photodiodes connected together by transfer transistors.
 6. Thephotodetector of claim 1, wherein the third region has a thickness suchthat it is an integral part of the space charge area between the firstand third regions.